Carbon fibrils are vermicular carbon deposits having diameters less than 500 nanometers (nm). They exist in a variety of forms, and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces.
Tennent, U.S. Pat. No. 4,663,230, describes carbon fibrils that are free of a continuous thermal carbon overcoat and have multiple graphitic outer layers that are substantially parallel to the fibril axis. As such they may be characterized as having their c-axes, the axes which are perpendicular to the tangents of the curved layers of graphite, substantially perpendicular to their cylindrical axes. They generally have diameters no greater than 0.1 micron and length to diameter ratios of at least 5. Desirably they are substantially free of a continuous thermal carbon overcoat, i.e., pyrolytically deposited carbon resulting from thermal cracking of the gas feed used to prepare them.
Fibrils are useful in a variety of applications. For example, they can be used as reinforcements in fiber-reinforced network structures or hybrid network structures, i.e., networks containing reinforcements such as continuous fibers in addition to fibrils.
In recent years, much interest has been expressed in the formation of molecular composites, i.e. composites in which individual molecules of rigid rod polymers are dispersed in more flexible matrix polymers to form mutually interpenetrating networks. It is generally believed that such composites will be able to withstand stresses substantially greater than conventional composites because stress will be distributed throughout the interpenetrating molecular system. Also, such composites will be less likely to suffer localized stress and will be able to withstand higher stress and/or strain before failure.
U.S. patent application Ser. No. 08/057,328 filed May 5, 1993 [PCT US94/04879, WO 94/25268], hereby incorporated by reference, discloses methods for forming three dimensional macroscopic assemblages of randomly oriented carbon fibrils. Broadly, dispersions of fibrils in a compatible liquid are prepared and the liquid is then removed to form a low-density porous plug or mat. In a preferred method, a low-density porous fibril plug is prepared by dispersing the fibrils in solvent, e.g., n-pentane, the dispersion is charged to a pressure vessel, the vessel, is heated above the critical temperature of the solvent, and the supercritical vapor is bled out of the vessel. In this manner a solid plug having the shape of the vessel interior is obtained.
U.S. patent application Ser. No. 08/857,383, filed May 15, 1997, hereby incorporated by reference, describes rigid, porous carbon structures comprised of carbon fibrils. The fibrils are bonded or glued to one another at their intersections. Bonding can be induced by chemical modification of the surface of the fibers to promote bonding, by adding "gluing" agents and/or by pyrolyzing the fibrils to cause fusion or bonding at the points of interconnection.